Various methods of determining fracture toughness values of metallic and ceramic materials have been established by the American Society of Testing Materials (ASTM) and these standard methods are widely accepted by the technical community. Accordingly, a wealth of test data obtained by such protocols has been reported and evaluated for many types of these materials. In spite of the adherence to these standard methods, the test data obtained can still be scattered and inconsistent even within a family of the same material type, resulting in irreconcilable test data. Differences in the size of specimens, the inhomogenity of the specimen material and other inherent specimen factors which are not standardized can result in such inconsistencies.
Additional difficulties in determining fracture toughness occur when evaluating weldments, which inherently consist of three different phase zones: weld; heat affect; and base material. Each of these zones is likely to manifest a characteristically different microstructure and mechanical properties. As is well known, the fracture behavior of the fusion line that lies between the solidified weld puddle and the heat affect zone still remains unexplored because of the lack of a standardized test method.
Each of these difficulties is further complicated when evaluating the fracture toughness of these materials for use in high pressure hydrogen environments. Such information is important and needed for many energy development programs, yet the influences of hydrogen on in-situ crack behavior of weldments are virtually unknown. The standardized or conventional testing protocols previously mentioned are neither physically suitable nor economically viable for in-situ testing in extremely high pressure hydrogen environments. ASTM recommended compact tension (CT) specimens, and their variations, are generally tested in open space, and are not tailored for in-situ testing in a controlled environment with an extremely limited space such as that which occurs in many desired applications for these materials. Small and thin CT specimens do not yield reliable data and are not effective for use in investigating fracture toughness or fracture cracking behavior of weldments. Accordingly, a spiral-notch torsion test system (SNTT) was invented by Jy-An Wang and Kenneth C. Liu, “FRACTURE TOUGHNESS DETERMINATION USING SPIRAL-GROOVED CYLINDRICAL SPECIMEN AND PURE TORSIONAL LOADING”, U.S. Pat. No. 6,588,283, the disclosure of which is hereby incorporated by reference, which utilizes a rod-type specimen having a helical groove with a 45-degree pitch to effectively simulate the fracture failure behavior of a thick CT specimen with a thickness equal to the total length of the groove line. This SNTT test method provides a small volume test specimen which is independent of the size effect previously encountered, and facilitates the testing of textured materials in any desired orientation.